Ellenburger Group Research Articles

Middle platform carbonate depositional systems and lithofacies patterns in the Lower Ordovician Ellenburger Group, Tobosa Basin in West Texas, U.S.A., and subsequent Sauk-Tippecanoe megasequence boundary meteoric karsting

A long, continuous core (1155 ft; 352 m) from the Ellenburger Group in Upton County, Texas allowed for a detailed reservoir description of depositional setting, lithofacies, pore networks, and impact of subsequent karst processes on the reservoir geology. This reservoir description furthers the understanding of the origin and characteristics of pore types relevant for waste-water disposal, carbon dioxide storage, and hydrocarbon reservoir quality in the Ellenburger Group. The Lower Ordovician Ellenburger Group in West Texas is a thick dolomite sequence that was deposited on a broad shelf over a 16 m.y. Time period in the Tobosa Basin. The shallow-water restricted depositional setting produced carbonate sediments that are dominantly composed of middle-shelf microbial thrombolite boundstones and inner-shelf laminated (stromatolitic) mudstones. Recognizable skeletal grains are rare. The lower two thirds of the cored section in the area of investigation in Upton County, Texas is dominated by microbial thrombolite complexes with associated stromatactis features. Interbeds of oolites and evaporites are associated with the complexes and may represent small-scale fluctuations in relative sea level. Up-section, lithofacies transition into a peritidal complex composed of stromatolites and laminated mudstones, some displaying evaporite nodules. The lithofacies stacking pattern is similar to the regional established stacking pattern where the most open-marine lithofacies are at the base of the stratigraphic section and the most restricted lithofacies are at the top of the stratigraphic section. This consistent stacking pattern suggests a stable structural setting for millions of years where constant environmental conditions existed. Following deposition and subsequent lithification and dolomitization of the Ellenburger Group, the section underwent intense meteoric karsting (epigenic) during Late Middle Ordovician Sauk-Tippecanoe megasequence long-term exposure. The meteoric karsting affected the upper 700 ft (215 m) of the Ellenburger Group. The pore network in the studied core is composed of vuggy (small cavities) pores in the thrombolite lithofacies and crackle and mosaic breccia fracture pores and chaotic breccias interbreccia pores in karst-affected intervals. These pore systems support hydrocarbon production, waste-water disposal, and carbon-dioxide storage.

Middle Platform Carbonate Depositional Systems and Lithofacies Patterns in the Lower Ordovician Ellenburger Group, Tobosa Basin in West Texas, U.S.A, and Subsequent Sauk-Tippecanoe Megasequence Boundary Meteoric Karsting

Middle Platform Carbonate Depositional Systems and Lithofacies Patterns in the Lower Ordovician Ellenburger Group, Tobosa Basin in West Texas, U.S.A, and Subsequent Sauk-Tippecanoe Megasequence Boundary Meteoric Karsting

Low pressure buildup with large disposal volumes of oil field water: A flow model of the Ellenburger Group, Fort Worth Basin, northcentral Texas

Low pressure buildup with large disposal volumes of oil field water: A flow model of the Ellenburger Group, Fort Worth Basin, northcentral Texas

Stratigraphic architecture and petrophysical characterization of formations for deep disposal in the Fort Worth Basin, Texas

Disposal of hydraulic fracturing flowback and produced water into Ordovician and Cambrian formations of the Fort Worth Basin (FWB), coupled with an increase in observed seismicity in the Dallas-Fort Worth area, necessitates an understanding of the geologic character of these disposal targets. More than 2 billion barrels (Bbbls) of wastewater have been disposed into the Ordovician Ellenburger Group of the FWB over the past 35 years. Since the implementation of the TexNet Earthquake Catalog (1 January 2017), more than 20 earthquakes of local magnitude ML2.0 or greater have been detected in the area, with depths ranging from 2 to 10 km (approximately 6500–33,000 ft). The cited mechanism for inducement of these earthquakes is reactivation of basement faults due to pore pressure changes, either directly related to proximal disposal or due to disposal volume buildup over time. Here, we present a stratigraphic and petrophysical analysis of FWB disposal targets and their relation to basement rocks. The Ellenburger consists of alternating layers of limestone and dolomite, with minor siliciclastics above the basement toward the Llano Uplift. Matrix porosity averages <5 porosity units (p.u.), with higher porosity in dolomitic layers than in limestone. Dolomite dominates at the top of the Ellenburger, which was exposed at the end of both the Lower and Upper Ordovician. Where crystalline basement rocks are penetrated, the composition ranges from granitic to chlorite-bearing metamorphosed lithology. The basement-sediment interface is frequently marked by increased porosity. An updated map of structure on top of basement indicates elevations ranging from outcrop at the Llano Uplift to more than [Formula: see text] ([Formula: see text]) subsea toward the northeast. The disposal zone pore volume is estimated from thickness and porosity maps and ranges from [Formula: see text] to [Formula: see text] billion barrels per square mile ([Formula: see text]).

Exploring the potential linkages between oil‐field brine reinjection, crystalline basement permeability, and triggered seismicity for the Dagger Draw Oil field, southeastern New Mexico, USA, using hydrologic modeling

AbstractWe used hydrologic models to explore the potential linkages between oil‐field brine reinjection and increases in earthquake frequency (up to Md 3.26) in southeastern New Mexico and to assess different injection management scenarios aimed at reducing the risk of triggered seismicity. Our analysis focuses on saline water reinjection into the basal Ellenburger Group beneath the Dagger Draw Oil field, Permian Basin. Increased seismic frequency (>Md 2) began in 2001, 5 years after peak injection, at an average depth of 11 km within the basement 15 km to the west of the reinjection wells. We considered several scenarios including assigning an effective or bulk permeability value to the crystalline basement, including a conductive fault zone surrounded by tighter crystalline basement rocks, and allowing permeability to decay with depth. We initially adopted a 7 m (0.07 MPa) head increase as the threshold for triggered seismicity. Only two scenarios produced excess heads of 7m five years after peak injection. In the first, a hydraulic diffusivity of 0.1 m2 s−1 was assigned to the crystalline basement. In the second, a hydraulic diffusivity of 0.3 m2 s−1 was assigned to a conductive fault zone. If we had considered a wider range of threshold excess heads to be between 1 and 60 m, then the range of acceptable hydraulic diffusivities would have increased (between 0.1–0.01 m2 s−1 and 1–0.1 m2 s−1 for the bulk and fault zone scenarios, respectively). A permeability–depth decay model would have also satisfied the 5‐year time lag criterion. We also tested several injection management scenarios including redistributing injection volumes between various wells and lowering the total volume of injected fluids. Scenarios that reduced computed excess heads by over 50% within the crystalline basement resulted from reducing the total volume of reinjected fluids by a factor of 2 or more.

Deep-burial dissolution and brecciation in dolostone reservoirs: Striking similarities between the Devonian Keg River Formation of western Canada and the Ordovician Ellenburger group of west Texas

Deeper burial dissolution of dolomitized fabrics created most of the reservoir quality in several Devonian Keg River oil pools on the Comet Platform in northwestern Alberta, Canada. This process also created reservoir quality in two Ordovician Ellenburger dolostone oil pools on the Eastern Shelf of the Midland Basin in west Texas, and provided an alternative explanation for reservoir development unrelated to the subaerial karstification model. Several observations confirmed deeper burial dissolution of Keg River dolostones. First, dolomites replaced, or cemented, grains previously sutured by pressure solution. Their subsequent dissolution postdated this pressure solution, confirming burial dissolution. Second, late-forming saddle dolomites dissolved. Third, secondary porosity developed along, or cross cut, stylolites, often in zebra dolostone fabrics, or formed along fracture planes that cut stylolites. These burial fractures often intersected secondary pores, implying a causal relationship. Fourth, dolostone breccia clasts contained stylolites rotated at different angles to each other and the horizon, confirming deeper burial brecciation. In the Keg River Formation on Comet Platform, proximity to conjugate reactivated basement faults that developed off of a master wrench fault, the Hay River Shear Zone, controlled deeper burial dissolution and pool entrapment. Hydrothermal fluids, either calcium rich or acidic in composition, were moved along these faults and promoted extensive dissolution of completely dolomitized fabrics. Ellenburger dolostone reservoirs at Suggs and Withers Fields on the Eastern Shelf of Texas also resulted from deeper burial dissolution of completely dolomitized facies. The Keg River dissolution fabrics were replicated in Ellenburger samples, with the exception of zebra dolostones. Ellenburger reservoir development was closely aligned to faults and fractures associated with a master wrench fault, the Ft. Chadbourne Fault. These faults were inferred to have moved hot (possibly hydrothermal?), deeper burial diagenetic fluids that were calcium-rich, or periodically acidic, promoting dolomite dissolution and secondary porosity development.

Attribute expression of fault-controlled karst — Fort Worth Basin, Texas: A tutorial

Much of seismic interpretation is based on pattern recognition, such that experienced interpreters are able to extract subtle geologic features that a new interpreter may easily overlook. Seismic pattern recognition is based on the identification of changes in (1) amplitude, (2) phase, (3) frequency, (4) dip, (5) continuity, and (6) reflector configuration. Seismic attributes, which providing quantitative measures that can be subsequently used in risk analysis and data mining, partially automate the pattern recognition problem by extracting key statistical, geometric, or kinematic components of the 3D seismic volume. Early attribute analysis began with recognition of bright spots and quickly moved into the mapping of folds, faults, and channels. Although a novice interpreter may quickly recognize faults and channels on attribute time slices, karst terrains provide more complex patterns. We sought to instruct the attribute expression of a karst terrain in the western part of the Fort Worth Basin, Texas, United States of America. Karst provides a specific expression on almost every attribute. Specifically, karst in the Fort Worth Basin Ellenburger Group exhibits strong dip, negative curvature, low coherence, and a shift to lower frequencies. Geomorphologically, the inferred karst geometries seen in our study areas indicate strong structural control, whereby large-scale karst collapse is associated with faults and where karst lineaments are aligned perpendicularly to faults associated with reflector rotation anomalies.

A window into the Proterozoic: Integrating 3D seismic, gravity, and magnetic data to image subbasement structures in the southeast Fort Worth basin

The Fort Worth basin (FWB) is one of the most fully developed shale gas fields in North America. Although there are hundreds of drilled wells in the basin, almost none of them reach the Precambrian basement. Imaged by perhaps 100 3D seismic surveys, the focus on the relatively shallow, flat-lying Barnett Shale objective has resulted in little published work on the basement structures underlying the Lower Paleozoic strata. Subtle folds and systems of large joints are present in almost all 3D seismic surveys in the FWB. At the Cambro-Ordovician Ellenburger level, these joints are often diagenetically altered and exhibit collapse features at their intersections. We discovered how the basement structures relate to overlying Paleozoic reservoirs in the Barnett Shale and Ellenburger Group. In support of our investigation, the Marathon Oil Company provided a high-quality, wide-azimuth, 3D seismic data near the southeast fringe of the FWB. In addition to the seismic volume, we integrated the seismic results with gravity, magnetic, well log, and geospatial data to understand the basement and subbasement structures in the southeast FWB. Major tectonic features including the Ouachita frontal thrust belt, Lampasas arch, Llano uplift, and Bend arch surround the southeast FWB. Euler deconvolution and integrated forward gravity modeling helped us extend our interpretation beyond the 3D seismic survey into a regional context.

3D seismic interpretation of dissolution pipes in the South China Sea: Genesis by subsurface, fluid induced collapse

A large suite of remarkable pipe structures has been identified from high-resolution 3D seismic data, in the Dongsha Massif, northern South China Sea. The diameters and heights of the collapse structures range from c. 100 m to 710 m and c. 134 m to 1010 m, respectively. These pipes vary from cylindrical to a steep conical geometry, narrowing upwards. They are rooted in a major carbonate succession that was originally deposited as a large carbonate platform system. These pipe structures are interpreted as resulting from carbonate diagenesis and dissolution at a deep burial stage, i.e. they are not classical karst collapse structures as suggested in previous studies. Instead they are analogous to pipe-like collapse structures described from the Ordovician Ellenburger Group of West Texas. The dissolution pipes formed at different times during the Neogene, with the earliest pipes developing during the early Middle Miocene (15.0 Ma) and with at least one peak of pipe formation during the Dongsha Event (began at 5.5 Ma). This regional event was associated with magma intrusion, uplifting and faulting in the study area. Stable isotope data of reservoir rocks in the area of pipe development suggest that cementation was linked to magmatically driven hydrothermal circulation during the Dongsha Event. Small-scale pipes are densely clustered above the main oil reservoir, and given that petroleum is known to have been generated during the time interval of pipe formation, we suggest that corrosive carbonic acid derived from flushing of petroleum reservoirs may have been a contributory factor to the dissolution and collapse process. (C) 2013 Elsevier B.V. All rights reserved.

Quantifying the origin and geometry of circular sag structures in northern Fort Worth Basin, Texas: Paleocave collapse, pull-apart fault systems, or hydrothermal alteration?

Three-dimensional seismic data reveal numerous subcircular sag structures in the northern Fort Worth Basin. The structures are defined by concentric faults that extend vertically upward 760–1060 m (2500–3500 ft) from the Lower Ordovician Ellenburger Group. The largest structures remained active into the lower Desmoinesian Strawn Group. Criteria are outlined for defining seismically resolvable sag structures, and a detailed quantitative analysis of the geometries of these circular features was undertaken. Results are compared and contrasted, with reviews of subsurface collapse mechanisms and strike-slip processes that are known to produce subsurface circular to subcircular sag geometries in plan view. In this manner, we develop several constraints for differentiating collapse-related sag structures from strike-slip–related sag structures. Qualitative analyses indicate that the geometries observed are strongly analogous to subsurface collapse features where material is removed at depth to create a void, into which the overburden subsequently sags and collapses. Quantitative analyses support the formation of these features by incremental collapse and suprastratal deformation above a linked system of coalesced, collapsed paleocaves within the Ellenburger Group. Observations and criteria presented herein provide valuable information in defining seismically resolvable collapse features worldwide and help distinguish sag features of collapse affinity from those of other origins.

Late opening-mode fractures in karst-brecciated dolostones of the Lower Ordovician Ellenburger Group, west Texas: Recognition, characterization, and implications for fluid flow

Two distinct groups of fractures in an Ellenburger Group reservoir in Barnhart field, Reagan County, west Texas, were identified. The oldest fractures (FBR) are the most numerous; have irregular shapes, sediment, and baroque dolomite fill, and no preferred orientation; and have been attributed by previous workers to brecciation associated with the collapse of Lower Ordovician paleocave systems. Younger, subvertical, opening-mode fractures (FY) that have consistent east-southeast and south-southwest strikes postdate the baroque dolomite cement. FY fractures therefore formed during the late stages or after the Pennsylvanian Ouachita orogeny. We analyzed FY fracture orientation, intensity, and openness using well image logs, oriented rotary-drilled sidewall cores, and a full-diameter core. FY fracture aperture sizes range from several micrometers to a few millimeters, and the fracture intensity is consistent within and between the wells studied at 1.8–4.0 102 fractures/mm2 for fractures 1 mm (0.04 in.) wide. Dolomite cement that is synchronous with FY fracture opening seals fractures in some locations, but is limited to fracture linings and mineral bridges in other places. Calcite, which grew after FY fractures stopped opening, is variably present and postdates dolomite cements. Where present, calcite occludes most remaining FY fracture porosity. Diagnosing the presence of postkinematic calcite is therefore an important step in being able to predict open fractures and was done for part of Barnhart field using rotary-drilled sidewall cores.

Three-dimensional architecture of a coalesced, collapsed-paleocave system in the Lower Ordovician Ellenburger Group, central Texas

The three-dimensional, interwell-scale architecture of a Lower Ordovician Ellenburger coalesced, collapsed-paleocave system was constructed through the integration of ground-penetrating radar (GPR), shallow-core, and outcrop data. The data were collected near Marble Falls in central Texas over an area (800 1000 m [2600 3300 ft]) that could cover several oil-well locations (160 ac; 0.65 km2) typical of a region such as west Texas. Integration of core-based facies descriptions with GPR-reflection response identified several paleocave facies that can be recognized and mapped with GPR data alone: (1) continuous reflections image the undisturbed strata, (2) relatively continuous reflections (over tens of meters) characterized by faults and folds image the disturbed strata, and (3) chaotic reflections having little to no perceptible continuity image heterogeneous, cave-related brecciated facies recognized in core that cannot be individually resolved with the GPR data. These latter facies include the highly disturbed strata, coarse-clast chaotic breccia, fine-clast chaotic breccia, and sediment fill.The three-dimensional architecture of the coalesced, collapsed-paleocave system based on core and GPR data indicates that there are trends of brecciated bodies that are as much as 350 m (1100 ft) wide, greater than 1000 m (3300 ft) long, and tens of meters high. These brecciated bodies are coalesced, collapsed paleocaves. Between the brecciated bodies are areas of disturbed and undisturbed host rock that are jointly as much as 200 m (660 ft) wide.As a cave system is buried, many structural features form by mechanical compaction. These features include folds, sags, and faults. The folds and sags measure from a few meters to several hundred meters wide. The collapse-related faults are numerous and can have several meters of displacement. Most are normal faults, but reverse faults also occur.

Characterization of a coalesced, collapsed paleocave reservoir analog using GPR and well‐core data

The three‐dimensional architecture, spatial complexity, and pore‐type distribution are mapped in a near‐surface analog of a coalesced, collapsed paleocave system in the Lower Ordovician Ellenburger Group near the city of Marble Falls in central Texas. The surface area of the site has dimensions of about 350 × 1000 m. The data collected include about 12 km of 50‐MHz ground‐penetrating radar (GPR) data arranged in a grid of orthogonal lines, 29 cores of about 15‐m length, and detailed facies maps of an adjacent quarry face. Electrical property measurements along with detailed core descriptions were the basis of integrated interpretation of the GPR data. Three main GPR facies are defined on the basis of degree of brecciation in the corresponding cores: undisturbed host rock, disturbed host rock, and paleocave breccia. This GPR facies division defined the major paleocave trends and the distribution of porosity types, which correlate with reservoir quality. Highly brecciated zones are separated by disturbed and undisturbed host rock. The breccia bodies that outline the trend of collapsed cave passages are up to 300 m wide; the intervening intact areas between breccias are up to 200 m wide. Understanding the breccia distribution in a reservoir analog will help in defining strategies for efficient development of coalesced, collapsed paleocave reservoirs.

ABSTRACT: Characterizing the Three-Dimensional Architecture of a Coalesced, Collapsed-Paleocave System in the Lower Ordovician, Ellenburger Group by Integrating Ground-Penetrating Radar, Shallow-Core, and Outcrop Data

ABSTRACT: Characterizing the Three-Dimensional Architecture of a Coalesced, Collapsed-Paleocave System in the Lower Ordovician, Ellenburger Group by Integrating Ground-Penetrating Radar, Shallow-Core, and Outcrop Data

Abstract: Results of the application of soft-computing techniques to characterization of a complex carbonate reservoir in the Ellenburger Group, Permian Basin, West Texas

Abstract: Results of the application of soft-computing techniques to characterization of a complex carbonate reservoir in the Ellenburger Group, Permian Basin, West Texas

Paleostructural and Related Paleokarst Controls on Reservoir Development in the Lower Ordovician Ellenburger Group, Val Verde Basin, Texas: ABSTRACT

Paleostructural and Related Paleokarst Controls on Reservoir Development in the Lower Ordovician Ellenburger Group, Val Verde Basin, Texas: ABSTRACT

Early- to Late-diagenetic Dolomitization of Platform Carbonates: Lower Ordovician Ellenburger Group, Permian Basin, West Texas

ABSTRACT Pervasive early- to late-diagenetic dolomitization of Lower Ordovician Ellenburger carbonates in the deep Permian Basin is recorded in core samples having present-day burial depths of 1.5 to 7.0 km. Petrography, geochemistry, and paragenetic relation of dolomite-rock textures indicate that Ellenburger carbonate rocks were subjected to a complex diagenetic history ranging from early shallow-subtidal dolomitization to late diagenetic deep-burial dolomitization. Fine-crystalline planar replacement dolomite of precursor lime muds formed during early diagenesis in a subtidal to peritidal setting under near-surface, low-temperature conditions, with Mg2+ supplied by diffusion from overlying seawater. During intermediate burial (500-2000 m), medium- to coarse-crystalline planar-s dolomite replaced allochems and matrix or occurred as void-filling. Planar-e dolomite precipitated along walls of pores and fractures or formed porous mosaics of medium- to coarse-euhedral crystals. Nonplanar-a dolomite replaced a precursor limestone/dolostone only in zones that are characterized by original high porosity and permeability. During deep-burial (> 2000 m) nonplanar dolomite cement (saddle dolomite), calcite, and authigenic quartz precipitated i and occluded fractures and pore-space. The nonplanar dolomite cement is typically enriched in calcium (mean = 52.6 mole % CaCO3) and iron (mean = 6635 ppm). Burial-history and thermal maturation calculations suggest that deep-burial dolomite cementation occurred during the Late Pennsylvanian/Early Permian. Inter- and intracrystalline dissolution surfaces are observed within the paragenetic sequence. Major truncation surfaces between early replacement dolomites and later void-filling dolomites, and between planar and nonplanar dolomite cements are evidence for dolomite dissolution. Deep-discharge of meteoric fluids as a result of frequent periods of karsting in overlying strata, and long-distance fluid migration during the Ouachita orogeny from foreland basins to the south are invoked for sources of undersaturated fluids causing dolomite dissolution and creating matrix-porosity in the deep subsurface. Dissolution of precursor dolomite may also provide a source of Mg2+ for late-stage dolomite cements. Similar diagenetic relationships have been described from deeply-buried carbo ate rocks elsewhere, indicating that trends and timing of dolomitization, dissolution and porosity formation, and cementation by late dolomite and calcite are intimately related to the evolution of sedimentary basins. The origin of massive dolostones, such as the Ellenburger, is best explained in the context of basin evolution, rather than by a single model of dolomite formation.

Dolomite‐rock textures and secondary porosity development in Ellenburger Group carbonates (Lower Ordovician), west Texas and southeastern New Mexico

ABSTRACTPervasive early‐ to late‐stage dolomitization of Lower Ordovician Ellenburger Group carbonates in the deep Permian Basin of west Texas and southeastern New Mexico is recorded in core samples having present‐day burial depths of 1.5–7.0 km. Seven dolomite‐rock textures are recognized and classified according to crystal‐size distribution and crystal‐boundary shape. Unimodal and polymodal planar‐s (subhedral) mosaic dolomite is the most widespread type, and it replaced allochems and matrix or occurs as void‐filling cement. Planar‐e (euhedral) dolomite crystals line pore spaces and/or fractures, or form mosaics of medium to coarse euhedral crystals. This kind of occurrence relates to significant intercrystalline porosity. Non‐planar‐a (anhedral) dolomite replaced a precursor limestone/dolostone only in zones that are characterized by original high porosity and permeability. Non‐planar dolomite cement (saddle dolomite) is the latest generation and is responsible for occlusion of fractures and pore space.Dolomitization is closely associated with the development of secondary porosity; dolomitization pre‐and post‐dates dissolution and corrosion and no secondary porosity generation is present in the associated limestones. The most common porosity types are non‐fabric selective moldic and vuggy porosity and intercrystalline porosity. Up to 12% effective porosity is recorded in the deep (6477 m) Delaware basin. These porous zones are characterized by late‐diagenetic coarse‐crystalline dolomite, whereas the non‐porous intervals are composed of dense mosaics of early‐diagenetic dolomites. The distribution of dolomite rock textures indicates that porous zones were preserved as limestone until late in the diagenetic history, and were then subjected to late‐stage dolomitization in a deep burial environment, resulting in coarse‐crystalline porous dolomites. In addition to karst horizons at the top of the Ellenburger Group, exploration for Ellenburger Group reservoirs should consider the presence of such porous zones within other Ellenburger Group dolomites.

Late-Stage Dolomitization of the Lower Ordovician Ellenburger Group, West Texas

ABSTRACT Petrography of the Lower Ordovician Ellenburger Group, both in deeply-buried subsurface cores and in outcrops which have never been deeply buried, documents live generations of dolomite, three generations of microquartz chert, and one generation of megaquartz. Regional periods of karstification serve to subdivide the dolomite into early-stage, which predates pre-Middle Ordovician karstification, and late-stage, which postdates pre-Middle Ordovician karstification and predates pre-Permian karstification. Approximately 10% of the dolomite in the Ellenburger Group is late-stage. The earliest generation of late-stage dolomite, Dolomite-L1, is interpreted as a precursor to regional Dolomite-L2. L1 has been replaced by L2 and has similar trace element, O, C, and Sr isotopic signatures, and similar cathodoluminescence and backscattered electron images. It is possible to differentiate L1 from L2 only where cross-cutting relationships with chert are observed. Replacement Dolomite-L2 is associated with the grainstone, subarkose, and mixed carbonate-siliciclastic facies, and with karst breccias. The distribution of L2 is related to porosity and permeability which focused the flow of reactive fluids within the Ellenburger. Fluid inclusion data from megaquartz, interpreted to be cogenetic with Dolomite-L2, yield a mean temperature of homogenization of 85 ± 6°C. On the basis of temperature/18Owater plots, temperatures of dolomitization ranged from approximately 60 to 110°C. Given estimates of maximum burial of the Ellenburger Group, these temperatures cannot be due to burial alone and are interpreted to be the result of migration of hot fluids into the a ea. A contour map of 18O from replacement Dolomite-L2 suggests a regional trend consistent with derivation of fluids from the Ouachita Orogenic Belt. The timing and direction of fluid migration associated with the Ouachita Orogeny are consistent with the timing and distribution of late-stage dolomite. Post-dating Dolomite-L2 are two generations of dolomite cement (C1 and C2) that are most abundant in karst breccias and are also associated with fractures, subarkoses and grainstones. 87Sr/86Sr data from L2, C1, and C2 suggest rock-buffering relative to Sr within Dolomite-L2 (and a retention of a Lower Ordovician seawater signature), while cements C1 and C2 became increasingly radiogenic. It is hypothesized that reactive fluids were Pennsylvanian pore fluids derived from basinal siliciclastics. The precipitating fluid evolved relative to 87Sr/86Sr from an initial Pennsylvanian seawater signature to radiogenic values; this evolution is due to increasing temperature and a concomitant evolution in pore-water geochemistry in the dominantly silic clastic Pennsylvanian section. A possible source of Mg for late-stage dolomite is interpreted to be from the dissolution of early-stage dolomite by reactive basinal fluids.

Early-to-Late-Diagenetic Dolomitization of Platform Carbonates: Lower Ordovician Ellenburger Group, Permian Basin, West Texas and Southeastern New Mexico

Pervasive early- to late-diagenetic dolomitization of Lower Ordovician Ellenburger carbonates in the deep Permian basin is recorded in core samples having present-day burial depths of 1.5 to 7.0 km. Fine-crystalline planar replacement dolomite formed during early diagenesis in a subtidal to peritidal setting under near-surface, low-temperature conditions, with Mg{sup 2+} for dolomitization of precursor lime mud supplied by diffusion from overlying seawater. During intermediate burial (500-2,000 m), medium- to coarse-crystalline planar-s dolomite replaced allochems and matrix, or occurred as void-filling. Burial-history and thermal maturation calculations suggest that deep-burial dolomite cementation occurred during the Late Pennsylvanian/Early Permian. Inter- and intracrystalline dissolution surfaces are observed within the paragenetic sequence. Major truncation surfaces between early replacement dolomites and later void-filling dolomites, and between planar and nonplanar dolomite cements are evidence for dolomite dissolution. Deep-discharge of meteoric fluids as a result of frequent periods of karsting in overlying strata, and long-distance fluid migration during the Ouachita orogeny from foreland basins to the south are invoked for sources of undersaturated fluids causing dolomite dissolution and creating matrix-porosity in the deep subsurface. Similar diagenetic relationships have been described from other deeply buried carbonate rocks elsewhere, indicating that trends and timing of dolomitization, dissolution and porosity formation,more » and cementation by late dolomite and calcite are intimately related to the evolution of sedimentary basins. The origin of massive dolostones such as the Ellenburger is best explained in the context of basin evolution, rather than by a single model of dolomite formation.« less